Eccentric shafts are made for a variety of uses. One of the most common uses is in internal combustion engines. In a piston-driven internal combustion engine the power is generated within a plurality of cylinders by reciprocating pistons which, depending on the combustion cycle employed, compress air or a combustible mixture of fuel and air for subsequent ignition. The pistons follow a reciprocating axial path, and are connected on a side opposite to their combustion face to connecting rods. The connecting rods are in turn connected to an eccentric shaft, the crankshaft. The crankshaft is used to translate the axial reciprocating motion of the pistons into rotational motion. The pressures generated by combustion in the cylinder acting through this rotational motion create the power output of the engine. Another eccentric shaft, the camshaft, is typically used in internal combustion engines to control the timing of the intake and exhaust valves in the cylinders.
Eccentric shafts are required to withstand both high torsional loading, as well as millions of load cycles. For this reason eccentric shafts are usually made of strong and ductile materials, such as steel, and are often hardened for added strength, either by cold working, or by heat treating, or by induction hardening the eccentric shaft to change the crystalline structure of the metal in the high load concentration areas to increase strength. The straightness of the eccentric shaft is critical to its operation, partly because it has to fit within the engine structure and partly because a lack of straightness can cause severe vibration. Straightness also gives the eccentric shaft good balance for rotation and reduces torsional vibrations.
An acceptable hardening process for certain internal combustion engines is roll hardening or cold working a crankshaft by rolling fillets on the edges of crankpin and main journal segments. However, in high output engines, particularly diesel engines, roll hardening may not produce sufficient crankshaft strength.
Induction hardening is a widely used process for the surface hardening of steel eccentric shafts. For example, a crankshaft is heated by alternating magnetic fields to a temperature within or above the transformation range of steel, followed by immediate quenching. The core of the crankshaft remains unaffected by the treatment, and its physical properties are those of the material it was initially formed in, but the hardness of the case is considerably increased by residual compressive stresses in the material, a result of quenching.
Eccentric shafts oftentimes may develop excessive run-out, or axial misalignment, partly as a result of residual stresses from the machining and induction hardening operations. In such cases, the run-out renders a part non-conforming to the eccentric shaft specifications, potentially resulting in scrap of a relatively expensive component. This is particularly important in a high volume production process because the material rejected increases cycle time and rework cost, as well as scrap rates.
The traditional method to straighten induction-hardened eccentric shafts is to straighten them using a press straightener to impart a load in a single plane to the eccentric shaft. However, the resulting deflection of the eccentric shaft may push a portion of the hardened case out of compression and into tension, thus locally lowering the strength of the shaft.
Accordingly, there is a need for straightening eccentric shafts, such as engine crankshafts and camshafts, and especially induction-hardened crankshafts and camshafts, without compromising their strength.